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If you round the degrees of freedom down to 13, The Rweb look-up of the t critical value is When using a computer! The computer happily handles non-integer degrees To round down to the nearest integer, which is wrong (even silly) MINITAB apparently follows the advice given in the book In the box on page 364 in Devore and Peck, which we can calculateĭf <- (vx + vy)^2 / (vx^2 / (nx - 1) + vy^2 / (ny - 1)) Is using the non-integer degrees of freedom defined by the formula The agreement is not exact, even allowing for rounding, MINITAB mustīe doing something wrong. This agrees more or less with the MINITAB output shown in the book. Type the URL above in the data URL window and submitĪnd Rweb returns, among other things, the 95% confidence interval Rweb has a function t.test that does t tests and Small-Sample Procedures for Means (Independent Samples)įor this section, we will use the data set for Problem 10.20 in Devore Get the answer in the back of the book for a homework problem, youĬan use Rweb as a calculator and for the "normal table look-up" with If for some reason you want the "large sample" procedure, perhaps to Hence, to do things the easy way, go to the The same thing for large n and be more conservative to boot. Use the t distribution for all procedures. The reason is that when using a computer, you might as well use the R has no simple procedure for doing "large sample" procedures for means. Large-Sample Procedures for Proportions Large-Sample Procedures for Means.Small-Sample Procedures for Means (Paired Samples).Small-Sample Procedures for Means (Independent Samples).Rweb in Stat 3011 Home Page Stat 3011 Home Page About the Rweb in Stat 3011 Web Pages Two Sample Tests and Confidence Intervals in Rweb (Stat 3011) Contents Two Sample Tests and Confidence Intervals in Rweb